1. Field of the Invention
The present invention relates generally to the protection of optical devices. More particularly, the present invention is directed to the protection of liquid crystal switches in optical networking elements.
2. Technical Background
A principal interest of communications technologies is enhancing networking ability. The primary goals behind each form of communication developed over the years were the improvement of transmission fidelity, the increase of data rates, and the increase of distance between relay stations. The speed at which light travels and its potential to address all of these concerns logically led to attempts at optical communication. Early experiments with optical communication suggested the feasibility of modulating a coherent optical carrier wave at very high frequencies, but were commercially impractical because of the installation expense and the tremendous cost of developing the necessary components. The combination of semiconductor technology, which provided the necessary light sources and photodetectors, and optical waveguide technology, however, eventually enabled the development and application of optical fiber-based systems despite these initially perceived difficulties.
In the recent rapid development of optical transmissions, networking has become commonplace and networking architectures have become increasingly complex. Most architectures now provide a large number of client network elements with the ability to both listen and transmit on optical channels within the network. Interconnected ring architectures provide even greater connectivity and complexity by bringing together client network elements of multiple rings. A relatively basic optical network will incorporate terminal multiplexers, add drop multiplexers, optical crossconnect systems, matched nodes, and interconnect nodes.
While certain networking components have been well defined, the design of many of these elements continues to undergo rapid development. Add drop multiplexers and ring interconnect nodes, for example, require optical switching ability and have only recently begun to make use of liquid crystal switches. A liquid crystal switch (LCS) is a switching device that modulates light which is already present. Such a device is presently being marketed by Corning Incorporated, the assignee of the present invention, under the name PurePath.TM. Wavelength Selective Switch (WSS). While LCS's provide high switching accuracy and are readily adaptable to the increasing complexity of optical networking structures, they are large and very expensive. A substantial complication associated with the use of LCS's in networking architectures is the possibility of device failure.
The use of any large and expensive component within an optical network presents questions of reliability. Essentially, when a working optical component fails, the network architecture must provide the necessary redundancy and switching to eliminate or minimize the potential for loss of communication between client network elements. Conventional approaches have placed a redundant protection LCS in series with a working LCS. There are a number of potential shortcomings, however, associated with this configuration. Such a structure, for example, requires complex coordination between the two switches and a detailed capability to sense the particular fault and its state. It is also important to note that if the defective LCS is partially switched, an effective open circuit is created and the series protection LCS cannot cure the defect. Placing an additional LCS in series also doubles insertion losses. Furthermore, this approach fails to solve the problem of loss of communication during repair of the defective LCS.
Flexibility, cost, and complexity of the protection system are also issues of concern. One-for-one redundancy therefore substantially adds to the cost of LCS systems which are already expensive. Shared protection is therefore highly desirable. Another issue associated with reliability is switching control for the protection device. For example, for a pair of four-port working LCS's it is desirable to detect failures via a mechanism other than detection of optical signal strength, because optical signal strength can be affected by a number of components throughout the network. Reliance on optical signal strength could therefore lead to unnecessary protection switching and inefficient network operation. Accordingly, it is desirable to provide a system and method for protecting optical devices which is not dependent on optical signal strength.